The present invention relates generally to the field of tamper protection and, more specifically, to the use of optically active, i.e., light emitting and detecting semiconductor and passive dielectric materials monolithically fabricated into a device incorporated into an enclosed sensitive region.
Tamper protection of sensitive equipment has two components: not only to protect against tampering, but also to detect if tampering has occurred. Given the assumption that the protected apparatus is actually in the possession and control of those who would be most likely to tamper with the apparatus, the originator of the device would have an interest in knowing whether tampering with the apparatus or information contained therein actually occurred. This device could be used for immediate sensing resulting in an alarm or penalty, to prevent tampering, but primarily, it is with the second interest of tamper detection that the invention is concerned. Here, the interest is in knowing whether tampering has occurred during some period of time.
There are optical devices which can detect motion and displacement, but these devices are not tamper detectors in the sense described above, because they are not capable of "remembering" that the motion occurred in the past. Thus, there is no means to prevent a tamperer from resetting the device to its original mechanical or electrical condition. Examples of this genera of devices include U.S. Pat. No. 4,560,868, entitled "Fiber Optical Luminescence Sensor for Measuring Mechanical Displacement," to Brogardh et al., U.S. Pat. No. 4,785,180, entitled "Optoelectronic System Housed in a Plastic Sphere," to Dietrich et al., U.S. Pat. No. 4,857,893, entitled "Single Chip Transponder Device," to Carroll, and U.S. Pat. No. 4,967,071, entitled "Fiber Optic Position Sensor," to Park et al.
The devices of Brogradh et al. and Dietrich et al. use individual optical components, such as LEDs, optical fibers, slit diaphragms, and photodetectors, and possibly these components can be substituted or replaced, which cannot be accomplished with an integrated device having all these functions monolithically fabricated onto a single substrate. In addition, rather than using waveguide coupling and absorption of light to detect tampering, Brogradh et al. use luminescence to detect motion and Dietrich et al. detect motion and displacement using the physical motion of a light beam over a photodetector.
The patent to Carroll also identifies an object, and has the same disadvantages as the motion sensors of Brogardh et al. and Dietrich et al. in relation to tamper sensing. The device of Carroll can detect if the identified object is moved out of the range of the transponder. Like Brogardh et al. and Dietrich et al., and Park et al., the Carroll device cannot "remember" that motion has occurred in the past if the object is returned to a location near its original position and there is no mechanism to prevent anyone from resetting the device to hide any evidence of motion in the past. In addition, Carroll uses radio frequency transmission and reception, digital logic, and digital signal processing to achieve the identification function, and the controller/interrogator unit is separate from the transponder. Park et al. use light reflection to detect motion, rather than waveguide coupling and absorption to detect tampering.
The device described in U.S. Pat. No. 4,840,281, entitled "Tamper Evident Optical Device and Article Utilizing the Same," to Phillips et al. does indicate when a package has been tampered with by relying on the selective absorption and reflection of particular colors of light. Moreover, the device by Phillips et al. has a mechanism to prevent the evidence of the tampering from being removed or erased. The light required by Phillips et al., however, is ambient room light, and the photodetector used is the human eye. It is designed for retail consumer packaging, and does not prevent replacement of the apparatus by a sophisticated tamperer.
It is thus an object of the invention to provide an optical tamper sensor to detect if tampering has occurred. This object is attained with the placement of the tamper sensor onto a critical entry plane of an enclosed sensitive region such that elements of the sensor will be irreversibly displaced when tampering occurs. This feature yields the additional advantage of the "pre-tampering" signal being impossible to duplicate.
It is an additional object of the invention to provide an optical tamper sensor which "remembers" that tampering has occurred. The feature of the invention related to this object is the analog or digital memory devices which have the stored signals representing the pretampering condition. An additional advantage is attained by this feature because the pretampering signals can be stored in more than one memory device, one of which can be physically removed from the enclosed sensitive region itself.
It is yet an additional object of the invention to provide a tamper sensor fabricated using integrated semiconductor technology. This object is achieved using optically active semiconductors, and using waveguide coupling and absorption. This feature, in turn, yields the significant advantages of being unobtrusive and easily hidden, as well as durable. An additional advantage is that the device cannot be replaced by another of the same construction because the absorption cannot be duplicated.
These and other objects of the invention are achieved with a tamper sensor having a light-emitting diode and a photodetector connected by a dielectric waveguide, all of which are monolithically fabricated from semiconductor and dielectric materials; the tamper sensor further has at least one absorber randomly placed on the surface of the waveguide, a voltage source to provide voltage to the light-emitting diode and the photodetector, and means for receiving and storing an output signal from the photodetector, wherein the output signal level is representative of a unique optical profile or signature obtained from the placement of the absorber on the surface of the waveguide. It is intended that the tamper sensor be placed onto a critical entry plane of an enclosed sensitive region such that entry into the enclosed sensitive region will displace the absorber from its position on the waveguide and change the optical profile, thus changing the output signal from its original stored value which value can be stored in a physical location removed from the enclosed sensitive region. It is further envisioned that the light-emitting diode is arranged at the center of a hub arrangement with a plurality of waveguides each having one photodetector at its outer end; each waveguide extending radially outward from the light-emitting diode. A plurality of absorbers of varying sizes would be randomly positioned with adhesive to the surface of the waveguides and would result in a unique optical profile or signature of the tamper sensor that would be impossible to duplicate or replicate.